Nuclear Instruments and Methods in Physics Research A 484 (2002) 613–618
Study of the neutron radiography characteristics for the solid
state nuclear track detector Makrofol-DE
Reynaldo Pugliesi*, Marco A. Stanojev Pereira
!
Divisao
* de F!ısica NuclearFTFF, Instituto de Pesquisas Energeticas
e Nucleares, IPEN/CNEN-SP, Caixa Postal 11.049FPinheiros,
CEP 05422-970 Sao
* Paulo, SP, Brazil
Received 14 June 2001; received in revised form 13 August 2001; accepted 17 August 2001
Abstract
In this work, the track-etch method was employed for Neutron Radiography purposes. A combination of the Solid
State Nuclear Track Detector Makrofol-DE with a natural boron converter screen has been used to register the image.
The radiography characteristics such as, track size, track production rate, characteristic curves and spatial resolution,
have been studied. The detectors were irradiated up to neutron exposures about 5 1010 n/cm2, in a radiography facility
installed at the IEA-R1 Nuclear Research Reactor, and etched in a KOH aqueous solution at a constant temperature of
701C. The obtained results were compared with those reported by other authors, and discussed according to the theory
of the image formation in solid state nuclear track detectors. The experimental conditions to obtain the best image
contrast, and the corresponding value of the spatial resolution, were also determined. r 2001 Elsevier Science B.V. All
rights reserved.
PACS: 81.70; 07.85.Y; 87.59.B
Keywords: Radiography; Non destructive testing technique
1. Introduction
The solid state nuclear track detectors (SSNTD)
are able to register some charged particles by the
radiation damage caused along their interaction
path in this material. These damages, under an
adequate chemical etching, are enlarged and called
tracks [1,2].
In the track-etch neutron radiography (NR)
method, a test object is usually irradiated in a
uniform neutron beam and a converter screen
*Corresponding author.
E-mail address: [email protected] (R. Pugliesi).
transforms the transmitted neutrons into ionizing
radiation, able to sensibilize the SSNTD. After
chemical etching (development), the resulting
tracks will form a two-dimensional image, which
is visible by naked eye. The main reasons that
makes these detectors attractive for NR purposes
are its insensitivity to register visible light, b and g
radiations as well as the high intrinsic spatial
resolution obtainable in the radiography image. Its
main disadvantage is the low optical contrast
achieved in the image [3].
The main objective of the present work was to
determine the experimental conditions to achieve
the best contrast in the image for the SSNTD
0168-9002/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 7 2 - 6
614
R. Pugliesi, M.A. Stanojev Pereira / Nuclear Instruments and Methods in Physics Research A 484 (2002) 613–618
Makrofol-DE, and to discuss its radiography
characteristics according to the image formation
theory in SSNTD, which is a model based on the
optical properties of a single track [3–6].
3. Results and discussion
The following radiography characteristics for
the Mk-DE detector were investigated:
3.1. Track size
2. Experimental
Table 1
Characteristics of the neutron beam at the irradiation position
Neutron flux
3 106 n/s cm2
Collimation ratio (L=D)
n=g ratio
Beam diameter
(Au)–Cd ratio
70
>105 n/cm2 mR
20 cm
B150
The image contrast as well as the spatial
resolution obtained in the radiography image are
usually influenced by the track diameter and so
they are etching time dependent. In general, small
tracks provide higher contrast and better resolution than the larger ones [3,9,10].
The track diameters have been evaluated as a
function of the etching time for a negligible track
overlapping condition. In the present work, it was
achieved for neutron exposure of about
xB9 107 n/cm2. Fig. 1 shows the results of the
track diameter behavior as a function of the
etching time.
Each experimental data has been obtained by
averaging the values of 10 distinct tracks, visually
determined onto the screen of the microscope TVmonitor system, by using a previously calibrated
length scale. The track diameters ranged from
1.4 mmoft o8 mm for etching times varying from
2ote o25 min.
According to the data reported in literature,
good quality radiographs can be obtained when
working with track diameters ft o5 mm. As shown
in Fig. 1, for the present detector such diameters
can be achieved for etching times te o15 min [7].
track diameter(µm )
The present NR facility is installed at the radial
beam-line 08 of the pool type IEA-R1 Nuclear
Research Reactor that operates at a power of
2 MW producing a thermal neutron flux of about
1013 n/s cm2 near the reactor core. The main
characteristics of the neutron beam at the irradiation position are listed in Table 1 [7].
The SSNTD Makrofol-DE (Mk-DE) is a
policarbonate 500 mm thick, manufactured by
Bayer AG [8]. The converter screen is a plastic
backing, single coated with a natural boron layer
having 106 and 65 mm thickness, respectively,
produced by Kodak Pathe" French. During irradiation, the detector and the screen are kept in a
tight contact inside an aluminum cassette. The
damages into the Mk-DE are induced by
the products of the nuclear reaction B10(n,a)Li7
(a-energy=1.47 MeV; Li-energy=0.84 MeV) and its
intrinsic registration efficiency is near the unity [5].
The chemical etching was performed in a PEW
solution (40 g ethanol, 45 g water, 15 g KOH) at a
constant temperature of 701C [7].
After chemical etching, the detectors have been
analyzed employing two systems. The first one was
a Leitz optical microscope (magnification power
1500 ) coupled to a TV-monitor to analyze
individual tracks and the second one was a
Jarrel-Ash optical microphotometer, having a
scanning beam width of 3 mm and length of
700 mm, to analyze light transmission through the
detectors.
10
9
8
7
6
5
4
3
2
1
0
0
3
6
9 12 15 18 21 24
etching time(min)
Fig. 1. Track diameter as a function of the etching time.
R. Pugliesi, M.A. Stanojev Pereira / Nuclear Instruments and Methods in Physics Research A 484 (2002) 613–618
3.2. Track production rate
3.3. Characteristic curve
A characteristic curve relates optical density,
Dop ; as a function of the neutron exposure. Optical
density is defined as ‘‘Dop ¼ logðIo =IÞ’’, where Io
2
track density(tr/cm )x10
6
3
2
1
0
0
5
10 15 20 25 30 35 40 45
etching time(min)
Fig. 2. Track density as a function of the etching time.
6
2
track density(tr/cm )
4x10
6
3x10
6
2x10
6
1x10
0
0
8
2x10
8
4x10
8
6x10
8
8x10
2
neutron exposure(n/cm )
Fig. 3. Track density as a function of neutron exposure for
6 min etching time.
1.4
1.2
optical density
This quantity represents the track to neutron
conversion efficiency.
It was determined for the etching time where the
efficiency is maximal. For such purpose one
detector has been irradiated at the exposure
x ¼ 3 108 n/cm2, and the track density evaluated
for several etching times in the interval from 2 to
40 min. Each track density value has been obtained
by averaging the amount of tracks counted in 10
distinct areas of the detector foil. The counting
was visually performed, onto the screen of the
microscope TV-monitor system. Fig. 2 shows the
track density behavior as a function of the etching
time and the efficiency is maximal for about 6 min.
In order to determine the track production rate,
the detectors have been irradiated in neutron
exposures ranging from 9 107 n=cm2 oxo
8 108 n/cm2 and etched in 6 min. The track
density was also determined by visual counting
and its behavior as a function of the neutron
exposure is shown in Fig. 3. The track production
rate was evaluated from the slope of the straight
line fitted to the experimental data points, and the
obtained value was tr=n¼ ð4:470:1Þ 103 [7].
615
1.0
0.8
0.6
0.4
0.2
0.0
7
10
8
9
10
10
10
10
2
neutron exposure(n/cm )
Fig. 4. Characteristics curves for three etching times: (’)
2 min; (K) 6 min; (m) 10 min.
and I are the intensities of the incident and
transmitted light through the detector, respectively.
In the present work, light intensity readings
have been determined by using the optical microphotometer. Fig. 4 shows the behavior obtained
for the optical density as a function of the neutron
exposures in the interval from 3 107 n/
cm2oxo6 1010 n/cm2 for three etching times 2,
6 and 10 min.
According to the image formation theory, a
track can be represented by an inner circle
surrounded by an external ring, which is responsible by the darkening of the image, as shown in
616
R. Pugliesi, M.A. Stanojev Pereira / Nuclear Instruments and Methods in Physics Research A 484 (2002) 613–618
Fig. 5. Schematic representation of a single track.
practical purposes is usual to employ the mean
value of G which is obtained from the slope of the
straight line fitted to the steeper region of the
characteristic curve [11]. The best value obtained
for the image contrast was G ¼ ð1:1070:01Þ for
etching times of 6 and 10 min. However, the
selected etching time was 6 min because, in this
case, the tracks are smaller and the dynamic range
is greater [7]. For this etching time the neutron
exposure
range
was
1.8 109 n/cm2oxo
10
2
3 10 n/cm .
It was also verified that for these same experimental conditions the fast neutron optical density
background, caused by (n; p) reactions is negligible. For such evaluation the detectors have been
irradiated without converter screen.
3.4. Resolution
Fig. 6. Schematic representation of two overlapped tracks.
Fig. 5. When the tracks are overlapped, see Fig. 6,
the external ring area will be smaller and consequently the optical density increases is slower.
Hence, the behavior of the characteristic curve for
etching time te ¼ 6 min can be explained as
follows: for neutron exposure up to xB1 109 n/
cm2, the track density is relatively low and,
consequently, insufficient to produce appreciable
optical density above the detector background
(Dop B0:04);
between
1 109 n/cm2oxo
10
2
3 10 n/cm , a competition between single track
production (responsible for optical density increase) and track overlapping (responsible for
optical density decrease) leads to a proportional
increase of the optical density. In this case, the first
process is overcoming the second one; for
x > 3 1010 n/cm2, track overlapping is predominant and the optical density decreases [3].
The
optical
contrast
is
defined
by
‘‘G ¼ dðDop Þ=dðlog xÞ’’ and its value varies for
each point of the characteristic curve [6]. For
In radiography, the spatial resolution is defined
as the minimum distance that two objects must be
separated before they can be distinguished from
each other [6]. The resolution is usually quoted in
terms of the total unsharpness (Ut ) and results
from the combined effect of the intrinsic unsharpness (Ui ) from the detector/converter screen
combination and of the geometric unsharpness
(Ug ) from the angular divergence of the neutron
beam and are related by the empirical equation
[12]:
ðUt Þn ¼ ðUi Þn þ ðUg Þn
ð1Þ
where 1ono3:
In the present work, the total unsharpness has
been obtained by scanning the optical density
distribution at the interface between two images:
the first one corresponding to a neutron opaque
knife edge test object (gadolinium foil 127 mm
thick, irradiated in a tight contact with the
detector) and the second one corresponding to
the direct neutron beam. The following Edge
Spread Function-ESF [13,14] was fitted to the
resulting distribution:
ESF ¼p1 þ p2 arctanðp3 ðX p4 ÞÞ
ð2Þ
where X is the scanning coordinate and p1 ; p2 ; p3
and p4 ; are free parameters. The total unsharpness
is given by ‘‘Ut ¼ 2=ðp3 Þ’’ [13].
617
R. Pugliesi, M.A. Stanojev Pereira / Nuclear Instruments and Methods in Physics Research A 484 (2002) 613–618
0.6
optical density
0.5
0.4
0.3
0.2
0.1
0
100
200
300
400
500
scann coordinate(µm )
Fig. 7. Optical density distribution for a gadolinium knife edge
test object and the fitted Edge Spread Function.
Table 2
Values of total unsharpness-Ut as a function of the neutron
exposure for etching time 6 min
Neutron exposure (n/cm2)
Ut (mm)
9
1.8 10
5.4 109
1.0 1010
3.2 1010
2172
2372
2272
3372
30
intrinsic unsharpness(µm )
Since the main disadvantage of the present
method is the low optical contrast achieved in the
image, the conditions of exposure range and
etching times for which the total unsharpness has
been evaluated were the same as the ones for which
the best image contrast was determined, that is,
1.8 109 n/cm2oxo3 1010 n/cm2 and 6 min, respectively.
The light transmission readings were performed
by using the same microphotometer and a typical
resulting optical density distribution as well as the
fitting of the ESF, are shown in Fig. 7. Table 2
gives the obtained results for Ut as a function of
the exposure for 6 min etching time.
The comparison of the present results with those
obtained from the theory of the image formation
in SSNTD was performed through the values of
the intrinsic unsharpness Ui : This theory states
that Ui is independent of the neutron exposure and
remains constant at a minimal value ‘‘Ui B0;
8 R’’ (R is the range of the a-particle in the
converter screen) for a negligible overlapping
condition and for track diameters ft oR: The
greater the exposure, the greater the track overlapping and for this condition ‘‘Ui a logðxÞ’’ [6].
Since for the screen used in the present work the
RB9 mm, the best value for the intrinsic unsharpness is theoretically Ui B7 mm.
For the present radiography facility L=DB70
and since the distance between the gadolinium foil
and the converter screen was 630 mm, the contribution of the geometrical unsharpness to the
25
20
15
10
5
0
9
10
10
10
2
neutron exposure(n/cm )
11
10
Fig. 8. Intrinsic unsharpness behavior as a function of the
neutron exposure for etching time 6 min.
total one was about Ug B9 mm. By using the values
found for Ut and Ug ; the intrinsic unsharpness can
be evaluated by means of Eq. (1) with n ¼ 1; 5 [14].
Fig. 8 shows the behavior of Ui as a function of
the exposure for 6 min etching time. These values
show a very similar trend when compared with the
ones obtained from the theory. The values remain
approximately constant at about 17 mm up to
exposure about xB2 1010 n/cm2 and increase
with ‘‘logðxÞ’’ when track overlapping becomes
predominant.
The discrepancy observed between the best
theoretical (7 mm) and experimental (17 mm) values
is attributed to the contact irregularities at the
detector/screen interface as well as to the neutron
beam and screen inhomogeneities. The former
leads to an a-particle scattering in the remaining
air layer at this interface and the latter to the
presence of small track clusters which have been
observed by microscope, even at low neutron
exposures [7].
618
R. Pugliesi, M.A. Stanojev Pereira / Nuclear Instruments and Methods in Physics Research A 484 (2002) 613–618
4. Conclusions
By taking into account the results obtained in
the characterization of the SSNTD Makrofol-DE
for NR purposes, the experimental conditions
to obtain the best contrast in the radiography
image are: 6 min for the etching time and
1.8 109 n/cm2oxo3 1010 n/cm2 for the neutron exposure range. For such conditions the
following characteristics were achieved: track
diameter 2.570.2 mm; track to neutron conversion
efficiency
4.470.1 103,
image
contrast
1.1070.01; total unsharpness from 2172 to
3372 mm.
The obtained results agree with those ones given
by the image formation theory for SSNTD which
states that the best contrast and spatial resolution
are achieved when the image is formed by many
small tracks instead by few large ones [3,11].
The main characteristic of the present method is
its high intrinsic unsharpness, which theoretically
can reach a value of about 7 mm. Although the
obtained experimental value is high, about 17 mm,
it can be promptly improved by using an
evacuated cassette. This characteristic together
with its insensitivity to register visible light, b
and g radiations, making the method useful for
example to obtain high resolution images for high
radioactive materials.
Table 3 gives comparative data for several NR
methods [11,15,16].
Table 3
Comparison of some characteristics for different radiography
methods
Converter
screen
Film/
detector
Irrad.
time (min)
Intrinsic
unsharpness (mm)
Gd
Dy
B(natural)
B(natural)
B(natural)
Kodak-AA
Kodak-AA
CN-85
Mk-DE
CR-39
5
10
120
120
120
70
400
12
17
13
The main disadvantage of the present method,
when compared with the one employing emulsion
films, is the low optical contrast achieved in the
radiography image. However, the employment of
standard digital processing techniques in the
obtained image, is overcoming this disadvantage
[7].
References
[1] M. Lferde, Z. Lferde, M. Monnin, et al., Nucl. Tracks
Radiat. Meas. 8 (1-4) (1984) 497.
[2] R.L. Fleisher, P.B. Price, R.M. Walker, Nuclear Tracks in
SolidsFPrinciple and Applications, University of California, Berkeley, California, 1975.
[3] R. Ilic, M. Najzer, Nucl. Track Radiat. Meas. 17 (1990)
453.
[4] R. Ilic, M. Najzer, Nucl. Track Radiat. Meas. 17 (1990)
461.
[5] R. Ilic, M. Najzer, Nucl. Track Radiat. Meas. 17 (1990)
469.
[6] R. Ilic, M. Najzer, Nucl. Track Radiat. Meas. 17 (1990)
475.
[7] S.M.A. Pereira, M. Sc, Thesis, Nuclear Energy National
Commission. IPEN-CNEN/SP Brazil, 2000.
[8] Application Technology Information, The Makrolon
3200. Bayer AG, 1997.
[9] R. Ilic, J. Rant, M. Humar, G. Somogyi, I. Hunyadi, Nucl.
Tracks 12 (1-6) (1986) 933.
[10] R. Antanasijevic, Z. Todorovic, A. Stamatovic, D.
Miocinovic, Nucl. Instr. and Meth. 147 (1977) 139.
[11] M.P.M. Assunc-a* o, R. Pugliesi, M.O. Menezes, Proceedings of the Fourth World Conference on Neutron Radiography, San Francisco, CA, USA, May 1992, pp. 10–16.
[12] M.R. Hawkesworth, Atom Energy Rev. 152 (1977) 169.
[13] M. Wrobel, L. Greim, Geesthacht, Germany, GKSS
(1988) (GKSS 88/e/12).
[14] A.A. Harms, A. Zellinger, Phys. Med. Biol. 22 (1) (1977)
70.
[15] R. Pugliesi, M.A.S. Pereira, M.A.P.V. Moraes, M.O.
Menezes, Appl. Radiat. Isot. 50 (1999) 375.
[16] M.O. Menezes, M.Sc. Thesis, Nuclear Energy National
Commission. IPEN-CNEN/SP Brazil, 1994.